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The assessment of horizontal force during overground sprinting is increasingly prevalent in practice and research, stemming from advances in technology and access to simplified yet valid field methods. As researchers search out optimal means of targeting the development of horizontal force, there is considerable interest in the effectiveness of external resistance. Increasing attention in research provides more information surrounding the biomechanics of sprinting in general and insight into the potential methods of developing determinant capacities. However, there is a general lack of consensus on the assessment and computation of horizontal force under resistance, which has resulted in a confusing narrative surrounding the practical applicability of loading parameters for performance enhancement. As such, the aim of this commentary was twofold: to provide a clear narrative of the assessment and computation of horizontal force in resisted sprinting and to clarify and discuss the impact of methodological approaches to subsequent training implementation. Horizontal force computation during resisted sleds, a common sprint-training apparatus in the field, is used as a test case to illustrate the risks associated with substandard methodological practices and improperly accounting for the effects of friction. A practical and operational synthesis is provided to help guide researchers and practitioners in selecting appropriate resistance methods. Finally, an outline of future challenges is presented to aid the development of these approaches.
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Assessing Horizontal Force Production in Resisted Sprinting:
Computation and Practical Interpretation
Matt R. Cross, Farhan Tinwala, Seth Lenetsky, Scott R. Brown, Matt Brughelli,
Jean-Benoit Morin, and Pierre Samozino
The assessment of horizontal force during overground sprinting is increasingly prevalent in practice and research, stemming from
advances in technology and access to simplied yet valid eld methods. As researchers search out optimal means of targeting the
development of horizontal force, there is considerable interest in the effectiveness of external resistance. Increasing attention in
research provides more information surrounding the biomechanics of sprinting in general and insight into the potential methods of
developing determinant capacities. However, there is a general lack of consensus on the assessment and computation of horizontal
force under resistance, which has resulted in a confusing narrative surrounding the practical applicability of loading parameters for
performance enhancement. As such, the aim of this commentary was twofold: to provide a clear narrative of the assessment and
computation of horizontal force in resisted sprinting and to clarify and discuss the impact of methodological approaches to
subsequent training implementation. Horizontal force computation during resisted sleds, a common sprint-training apparatus in the
eld, is used as a test case to illustrate the risks associated with substandard methodological practices and improperly accounting for
the effects of friction. A practical and operational synthesis is provided to help guide researchers and practitioners in selecting
appropriate resistance methods. Finally, an outline of future challenges is presented to aid the development of these approaches.
Keywords:sprint training, heavy sled, system mass, friction coefcient, kinetics, ground-reaction force, power
A key physical determinant of many sports is the ability to
express force at a range of movement velocities.
In sprinting
acceleration, assessing the component of ground-reaction force
applied in the direction of the sprint (ie, horizontal force [F
]) can
help further our understanding of performance. As such, there is
considerable interest in assessing F
with applications in monitoring
and rehabilitation,
as well as the selection of training parameters.
One area of interest is assessing F
while sprinting against
resistance to better understand the kinetic impact of potential
training regimes. Unfortunately, measuring F
during resisted
sprinting (eg, towing a sled or using a winch device) is somewhat
complex, and ensuring accurate and consistent measurement and
computation is crucial. The aim for this commentary is to provide a
clear narrative of the assessment and computation of horizontal
force in resisted sprinting, and to critically discuss the impact of
methodological approach on training applicability.
Interplay of Mechanics
During Sprinting Acceleration
During sprinting acceleration, the force produced by muscle aims
to overcome inertia and any friction forces. The greater the force
(in this case F
) expressed, the greater the acceleration for a given
system mass (body mass plus any additional load; see Table 1).
Any external resisting forces impede acceleration for a given
output and must be overcome if the athlete is to accelerate.
In a typical sprint, no resistive forces are present except air friction
force (F
), which increases with the square of velocity (ie, high at
the end of the acceleration phase).
In resisted sprinting, both
inertia and resistive force can be manipulated.
In a resisted sprint, the same principles can be applied in the
horizontal direction as unresisted sprinting.
The main differences
are that (1) system inertia typically increases (eg, resisted sleds)
and, with it, the force required to accelerate, and (2) additional
resistive forces are presentthe magnitude and manner of which
are method dependent (commonly sliding friction force); practi-
cally, the result is less acceleration and a lower peak velocity
attained per unit of F
. However, while system inertia is simple,
computing resistive force is more complex.
Measurement and Computation
of Horizontal Force
Measurement techniques fall into two categories: direct and indi-
rect. The former involves the direct assessment of ground reaction
forces using force platforms embedded under the running surface
(typically in the ground
or under a treadmill belt
). From this point,
can be separated from the resultant ground reaction force vector.
can also be indirectly estimated using a strain gauge attached
between the athlete and a xed point, or resistive device.
In this
case, the raw output is an offset force,which makes it difcult to
precisely estimate F
specic to ground contact (a limitation of
various force treadmills
). Other indirect methods estimate net F
production over time by utilizing an inverse dynamic approach
applied to the center of mass.
The approach requires only velocity
Cross and Samozino are with the Interuniversity Laboratory of Biology of Motricity,
Savoie Mont Blanc University, Chambéry, France. Cross is with the Scientic and
Sports Dept, Fédération Française de Ski, Annecy, France. Cross, Tinwala, Le-
netsky, Brown, Brughelli, and Morin are with the Sports Performance Research Inst
New Zealand (SPRINZ), Auckland University of Technology, Auckland, New
Zealand. Brown is with Neuromuscular and Rehabilitation Robotics Laboratory
(NeuRRo Lab), Dept of Physical Medicine and Rehabilitation, University of
Michigan Medical School, Ann Arbor, MI. Morin is with the Côte dAzur
University, LAMHESS, Nice, France. Cross ( is
corresponding author.
International Journal of Sports Physiology and Performance, (Ahead of Print)
© 2019 Human Kinetics, Inc. INVITED COMMENTARY
or distancetime measurements and has been used on data from a
variety of devices (eg, global positioning systems,
radar, laser,
photovoltaic cells,
and high-speed cameras
While the former direct approach can provide insight into the
specic events occurring, the advantage of the latter indirect
approach is that it can more easily be integrated into the eld.
The effects of resistance on F
can certainly be measured using
direct approaches (typically treadmills
or single foot strikes
), but it is for reasons of practicality that indirect
approaches have attracted interest.
Methods of Manipulating F
Resistance in Sprinting
There are four main methods of providing horizontal resistance
to an athlete: aerodynamic (eg, parachutes), motorized (otherwise
termed robotic;eg, 1080 Sprint; 1080 Motion, Austin, TX),
pulley (eg, Exergenie, Thousand Oaks, CA), and sliding (eg, sleds
or equivalent). While not well examined, aerodynamic resistance
should adhere to the same drag computations published on the
human body (ie, F
; permitted shape and drag factor are known).
Contemporary motorized devices provide resistance via measured
increments of torque via a servo motor.
Force production will
typically be computed as a product of simulated inertia, resistive
value of braking, and acceleration via rotary encoder. Pulley
devices provide resistance in the form of friction applied to a
drum or directly to a cable in more simple devices, and do not
substantially increase the inertia of the system. However, the
magnitude and consistency of resistance provided per increment
of braking must be experimentally determined. Sliding devices
move with the athlete, and consequently increase system inertia.
Additional resistive force is also present from the sliding of the
device over the ground (ie, friction force [F
In its simplest form, F
can be estimated using a basic
conversion factor (coefcient of friction;μ
); determined
experimentally means approximated based on known values
(eg, normative data on the coefcient between metal and grass).
represents the conversion between the normal force the sled load
applies to the ground (ie, the sled + additional load under gravity)
and the resulting horizontal F
applied to the athlete. For example,
if sled sprinting with a sled mass of 40 kg and a μ
of 0.3, F
be approximately 128 N. μ
can change depending on the surface
device characteristics, magnitude of normal loading, environmen-
tal characteristics, and even movement velocity.
One can estimate
using a common handheld baggagescale to directly measure
the friction force across a range of masses by pulling at a constant
speed (tting a linear regression between pulling force and normal
however, substantially more complex methods might be
necessary to provide accurate results.
Interested parties are
encouraged to read further.
At present, resisted sleds are probably the most commonly
used external horizontal overload,
and for this reason (and their
relative computational complexity), we will use this method to
focus the following discussion.
Problems Arising
From Inaccurate Computation
A clear problem in the literature is a lack of narrative regarding the
type and magnitude of resistive forces (generally, friction) provided
by resistance modalities and, consequently, how this is factored into
computations of total output. Notably, a recent meta-analysis
classied loading solely on the additional normal load (% body
mass), and while surface typeswere reported (eg, rigid), this
rudimental categorization does not displace or accurately represent
friction characteristics. The overarching problem is that, for a given
load, the resistance experienced by the athlete could change sub-
stantially (eg, doubled or halved
) purely as a product of
friction characteristics.
To illustrate the problem with a lack of consensus of approach,
we will compare methods of two studies
that determined
indirect, external F
using resisted sleds. Both studies aimed to
compute the optimal loading for maximizing horizontal power, and
were published in quick succession, but provided markedly differ-
ent results and conclusions. The rst is a study by Cross et al
in which F
was computed at peak velocity (an approach justied
and discussed at length
) following a friction experiment.
second is a study by Monte et al,
who computed F
the acceleration phase of each sprint while noting kinematic
The base computations (per unresisted sprinting) used in both
studies are identical, but they differ in accounting for resistance due
to external loading. Cross et al
computed F
by increasing the
total inertia of the system per the mass of the sled and harness, and
Table 1 Nomenclature, Basic Denitions, Measurement, and Computational Background of Horizontal Force
Definition Measurement and calculation
Portion of the ground-reaction
force acting in the sprint direction
Measured directly at the point of application using force platforms. Otherwise, estimated as
the product of its determinant variables.
Acceleration occurring in the
direction of the sprint
Determined as rate of change in velocity with respect to time. Can be measured via a wide
variety of technologies to various degrees of accuracy.
mTotal system mass Measured using a simple summation of all mass included in the system being accelerated
(eg, body mass, vest, loaded sled).
Aerodynamic friction force Estimated using published equations, from a combination of athlete characteristics (mass
and height, converted to frontal area), environmental characteristics (temperature, wind
speed, and pressure), and instantaneous velocity.
Friction force Estimated using experimentation or published equations. Magnitude is determined by the
application of braking or loading, converted to effective resistance using a coefcient of
friction. The coefcient may not be consistent and can change based on loading and/or
velocity. Final value may require correction for angle of pulling (which itself can be
measured directly or estimated from standing height and tether length).
Base computation: Fh=m·ahþFaero þFf:
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2Cross et al
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including F
as a constant (per F
). F
was computed using a sled
load and an experimentally conrmed μ
, which varied based on
velocity (via a polynomial t).
The angle between the ground and
tether was also calculated to help separate the horizontal ground
reaction force component from any vertical occurring. Monte
et al
did not directly compute friction force, but considered
the additional resistance as an equivalent load experienced by
the athlete. To correctfor the effects of friction, the initial
prescription of each sled mass was reduced by the magnitude of
frictionequivalent loading using an experimentally determined μ
of 0.2 (eg, original mass = 10 kg, equivalent loading = 2 kg, nal
mass prescribed = 8 kg). In this manner, it appears that the authors
believed that the effects of friction force were completely accounted
for, although it remains unclear if the reduced or original load was
used in F
computations. Clearly, correcting for horizontal load-
ingby subtracting its raw magnitude from that applied vertically is
problematic, since F
is a product of the total normal load of the sled
and environmental characteristics (eg, 1.6-kg-equivalent loading is
still experienced for a corrected8-kg sled mass).
The method presented by Monte et al
may result in substan-
tial inaccuracies in the force produced to overcome inertia during
acceleration (considered mass may differ from that experienced)
and an underestimation of friction forces (notably in late sprint
phases). To facilitate understanding, Figure 1displays the effects of
this method on computed F
during sprinting acceleration with two
loads. Notably, the limitations of this method are perhaps most
apparent in conditions nearing zero acceleration (eg, peak or
maintained velocity), since the force to overcome sled friction is
correctedfor in an articial inertia. Consequently, the computa-
tion assumes at constant velocity the only net F
being produced is
to overcome F
. To emphasize, regardless of whether the sled
load is 5 kg or 100 kg, the force produced by the athlete during
constant velocity is assumed to be only due to air.
Implications for Training
The F
output during resisted sprinting is regularly presented as a
means of understanding the value of potential training parame-
; the examples earlier illustrate how methodological prob-
lems can directly affect the results on which such judgments are
made. Similarly, the methods through which mechanical parame-
ters are measured and computed need to be associated with a clear
narrative around their specic applicability.
Figure 1 Illustration of the differences in external maximal power in the horizontal direction over the course of 2 sprints, with (black line) and
without (gray line) accounting for the constant variable of friction force, applied to the instantaneous velocitytime data of athletes sprinting with 2
different loading protocols: (A) 20% body-mass load (chosen to be representative of the highest loading protocol used by Monte et al
) and (B) 79%
body-mass load (corresponding to approximately the mean load maximizing power in Cross et al
). Black line indicates computation accounting for the
effects of friction force as a constant
; gray line, correction computation used by Monte et al.
Both examples are based on the same calculation of
inertia, on a Mondo track surface, coefcient of friction = 0.4.
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Horizontal Force in Resisted Sprinting 3
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Commonly, measurement occurs under a selection of external
protocols, after which instantaneous or average F
outputs are
compared with gain insight into which parameters acutely enhance
output (eg, maximizes force or power). However, the nature of such
measurement and computation may be problematic for those
expecting benecial training outcomes. As an example, if training
resistance is selected based on instantaneous maximization during
an athlete will similarly spend only a single instant
in these targeted conditionsper acceleration performed. The
minimal cumulated time spent in targeted conditions per training
block might limit practical benet and could partially explain the
typically similar adaptations experienced in resisted versus unre-
sisted sprinting interventions.
A clear solution to increasing exposure is to nd the load that
allows targeted conditions to be reached at the peak velocity plateau,
since this is the only velocity that can be maintained over several
seconds of maximal exercise (fatigue permitting).
The inter-
est in this approach is that if resistance is selected to target the
development of a given level of F
at peak velocity, the athlete can
spend an extended period (eg, 35 s per sprint) near these optimized
conditions by simply attaining and maintaining peak velocity. The
practical consequence is greater time spent in targeted conditions
than other approaches (see Figure 2for an example of targeting
maximum power). Nevertheless, the method has limitations; namely,
it ignores the work performed (and fatigue induced) between the start
and reaching peak velocity. An alternative approach might be the
assessment of loading parameters based on the average output over a
distinct period (eg, 10 m). Certainly, this is a valuable avenue for
future investigation.
Distinct bands of velocity can be targeted by xing a specic
training load, and these conditions are likely relevant to unloaded
While some may argue that loading selected in this
manner ignores technical specicity because it aims to recreate
the conditions experienced during a specic moment of the acceler-
ation phase, it follows that kinematic conditions might be similar
when compared in this manner (eg, the instant of 5 m·s
acceleration vs 5 m·s
peak resisted velocity). These similarities (or
indeed dissimilarities) require clarication, but do not necessarily
degrade the possibility of longitudinal improvement in determinant
physical capabilities as part of a balanced periodization. Neverthe-
less, the actual effects of training using stimuli selected for its kinetic
specicity remain unclear,
and more research is needed.
Practical Synthesis
We encourage researchers to utilize the measurement of F
resisted sprinting but to carefully consider the impact of study
design and computation on their conclusions. Where possible,
Figure 2 Graphic showing the differences in (A) power output and (B) F
over the course of 2 different loading protocols: unloaded sprinting (black
line) and 79% body-mass load (gray line). The computation of force for the resisted sprint follows the procedures described in detail elsewhere.
This is
intended to exemplify the difference in time spent at a targeted kinetic output (in this example, maximum horizontal power), when assessed at maximum
resisted velocity compared with during the acceleration phase.
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4Cross et al
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experimentally determine friction coefcients, or consider using an
outcomevariable (eg, decrement in maximum velocity), to select
and classify loading parameters. Practitioners may use published
coefcients to estimate outputs, while acknowledging substantial
errors may occur. Those wishing to use resisted sprinting to target
the development of distinct bands output might forgo F
ment and simply select loading based on velocity decrement.
Where training implications are valued as an outcome, researchers
are encouraged to carefully orient their study design and analysis to
maximize applicability to practice, or at least clearly discuss
limitations. Finally, the measurement and improvement of F
does not supplant a balanced approach to improving sprinting
performance and, therefore, should not be presented nor interpreted
as such by researchers and readers alike.
Rapidly developing technology has made F
assessment feasible
for many practitioners. Assessing F
can provide valuable insight
into athlete capabilities, and can guide training when combined
with resisted sprinting. This progress is accompanied with a need
for careful implementation and computations. As such, researchers
must be attentive in their methodological proceedings and well
consider the practical applicability of their ndings.
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Horizontal Force in Resisted Sprinting 5
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... For example, specialized sprint running treadmills provide a practically unparalleled assessment of horizontal force-production characteristics, yet, because few practitioners have access to such equipment, more practical field methods have been developed and widely adopted (Cross et al., 2019). In the following section, we will briefly outline the current stance on assessing external force production of the lower limbs in different muscle actions using field methods. ...
... Therefore, to increase this force, a skier can theoretically increase the lateral angulation with the snow and increase the resultant normal reaction force (i.e., the normal reaction force applied to the ski in the ski reference, nSkiRF). The positive correlation between mean nSkiRF value and performance is shown by some studies on direct and indirect measurement (Cross et al., 2019). Moreover, the application of nSkiRF differs with turn trajectory, radius (Nakazato et al., 2011), and skier level . ...
... Overlay of the force output of the lower limbs of a skier and a sprinter. For the skier (BW=90 kg; dashed line), data corresponds to the course averaged output per limb (inside=grey, outside=black; solid line) from one turn as measured by specialised force-platforms from a raw dataset (Cross et al., 2019), showing both feet independently. For the sprinter (BW=72 kg), footstrikes (i.e., impulse followed by flight time and subsequent impulse of opposite limb) 2-7 of a maximum sprint are displayed (limb 1=black, limb 2=grey), as measured by in-ground force-plates from a raw dataset (Morin, Samozino, Murata, Cross, & Nagahara, 2019). ...
Full-text available
The literature characterizing force production during skiing, and the associated capacities of skiers, is complicated to synthesize due to ageing results or relatively unspecific assessments. The overarching aim of this doctoral thesis was to clarify the importance of force-output for skiing and of specific force-production capacities for different disciplines. The thesis comprised two themes: 1) characterizing the force-output of skiers (N=15) on a giant-slalom course using kinematic and kinetic data from a global positioning system and boot-mounted force-platforms, respectively; and 2) measurement of dynamic and isometric force, the effect of countermovement on force production at different velocities, and specific strength-endurance across disciplines, and performance levels, in national skiers (N=31) and sprinters (N=30, for comparisons). The conclusions from Theme 1 were that radial force-output applied to turn the skis was linked with performance (R2=0.31–0.68, p<.032), and depended on both total magnitude and the ability to apply the force effectively (β=0.63–1.00, p<.001). A high total force magnitude was associated to high force production by both the outside and inside limbs (β=0.92–1.00 and 0.631–0.811, respectively, p<.001). For Theme 2, athletes from speed and technical disciplines displayed different dynamic and isometric force qualities, with the former showing superior dynamic force at low velocities (ω2=0.17, p<.001) and in isometric conditions (ω2=0.16–0.22, p<.003). Overall, performance was linked with a more force-dominant profile (ω2=0.34; r=-0.60– -0.67, p<.001) and increased rate of force development characteristics (r=-0.50– -0.82, p<.048). Robust associations existed between maximum isometric force and speed discipline performance (r=-0.88, p<.001), but tended to be for technical athletes (r=-0.49, p=.052). Force production at moderate velocities did not separate disciplines, nor was it associated with performance. Variability in the shift of mechanical characteristics and inverse correlations between force augmentation at different velocities (rs=-0.74, p<.001) indicated countermovement effect depended on extension velocity. Skiers exhibited a smaller countermovement effect at low velocities (rrb=-0.68, p<.001), with the opposite observation for sprinters (rrb=0.43, p=.008). ‘Moderate’ velocities failed to differentiate groups. Better skiers produced greater force at low speeds with a smaller countermovement effect, which supports the existence of velocity-specific strength qualities. The ski-specific strength-endurance assessment yielded some discriminative results, but, due to interactions between the test settings and real athlete capacities, the principal value of this section was to direct future protocol design. This thesis generally supports the assertion that force output during skiing is partly limited by force production capacities. On snow, both high force-output capacity and effectiveness of application were associated with performance. Off snow, better-ranked athletes possessed the highest capacity for specific force-production capabilities. High-level skiers appear to display a dominance of force-production at low speeds and in isometric conditions compared to other sports.
... Producing greater horizontal force is proven to improve sprint acceleration performance, whereas directing braking forces to be vertical and propulsive to be horizontal optimises the constant velocity phase of a sprint [13]. Thus methods such as sled towing have been used to target horizontal force production and accelerative sprint ability [14]. ...
... sleds) [12,14]. The reason why this training modality is popular is because it overloads the running action and provides a horizontal stimulus. ...
... The tools and modalities that deliver sprint-specific training stimuli are many, one of the most commonly used tools being resisted towing devices (e.g. sleds) [12,14]. It appears that resisted sled towing (RST) can improve horizontal force production, which is linked with enhanced sprint performance, especially during the acceleration phase of the sprint [14,32,66]. ...
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The success of many team sports and track and field athletes can be in part linked with their sprint performance. Therefore, improving sprint performance has been the foci of researchers and practitioners alike. The most commonly used tools that deliver sprint-specific training stimuli are resisted towing devices (RST) (e.g. sleds). RST provides a predominantly concentric (CON) horizontal overload to the musculo-skeletal system, especially in the acceleration phase of the sprint. Perhaps an eccentric (ECC) horizontal overload may be beneficial given the benefits of ECC training; such as, injury prevention and rehabilitation, shift towards faster muscle phenotypes, hypertrophy, strength and power improvements. This resulted in the overarching research question, “Can a novel horizontal ECC towing device improve sprint performance?”. The aim of this thesis was to develop a device that would provide a horizontal ECC stimulus, evaluate the biomechanics of the device and test its effects on sprint performance. A review of existing ECC training devices found limited devices overload in the horizontal plane and none eccentrically overload the musculo-skeletal system in a sprint-specific gait. Therefore, a movement termed horizontal ECC towing (HET) was developed which involves an athlete in a sprint stance trying to move forwards but is being pulled backwards. A device termed the HET device was then developed to automate this movement. The device was powered by a 10 kW electric motor that can tow athletes at velocities up to 3.58 m/s and can tolerate forces up to 2.8 kN. Two familiarisation sessions were found to achieve movement consistency during HET. Biomechanics analysis was conducted to further understand the movement which would help inform training programme development for coaches. Since HET is a novel movement, no research existed. Thus, ECC towing was compared to its opposite, the CON towing direction (CTD). Statistical Parametric Mapping (SPM) analysis of ground reaction force (GRF) profiles found that the two directions were significantly different (p<0.05) and were applying different movement strategies to produce force. This suggested that different lower limb joints were likely responsible for CON and ECC force production. Vertical and horizontal GRFs were lower in the ECC direction (p<0.05), which may be limited by the coefficient of friction and indicated that isokinetic horizontal towing does not follow the contractile-force-velocity relationship. Power and work analysis of the lower limb joints showed that the ankle and hip joints are absorbing energy and likely dissipating it in the ECC towing direction (ETD). ETD has greater ankle and hip joint power absorption and much smaller power production. A four-week intervention of ECC and CON towing in elite female field hockey players (n=10) resulted in no improvements in split times. There is still an opportunity for practitioners and researchers to apply a unique ECC stimulus to their athletes. The intervention study had its limitations as it was based out of the lab in a practical setting. However, no tool provides a similar overload as the HET device. We recommend to those that are interested in overloading the power absorption phase of the ankle and hip joints should incorporate HET. Further research with the HET device involving a larger cohort of athletes could provide more conclusive evidence on the effects on sprint performance.
... In this method, where the resistance usually is created by towing a sled, the user can target the development of various sprint phases by increasing or decreasing the load. 11 This loading represents a continuum, with heavy loads roughly corresponding to horizontal force at low speeds, early sprint phases, and short distances and lighter loads corresponding to horizontal force at high speeds, late acceleration (or perhaps maximum velocity), and long distances. 12 The RST with light loads (∼10% body mass [BM] or ∼10% reduction of maximal speed [v dec ]) has traditionally been studied and recommended for improving sprint acceleration. ...
... This method characterizes the "optimal load" (L opt ) as that which allows P max to be reached during the maximum resisted velocity plateau (ie, 50%v dec ) and thus maintained for longer than a single instant during training. 11,19,24 Consequently, L opt represents the loading at which the athletes can maximize the time spent in conditions close to maximum horizontal power. ...
Purpose: This study compared the effects of heavy resisted sprint training (RST) versus unresisted sprint training (UST) on sprint performance among adolescent soccer players. Methods: Twenty-four male soccer players (age: 15.7 [0.5] y; body height: 175.7 [9.4] cm; body mass: 62.5 [9.2] kg) were randomly assigned to the RST group (n = 8), the UST group (n = 10), or the control group (n = 6). The UST group performed 8 × 20 m unresisted sprints twice weekly for 4 weeks, whereas the RST group performed 5 × 20-m heavy resisted sprints with a resistance set to maximize the horizontal power output. The control group performed only ordinary soccer training and match play. Magnitude-based decision and linear regression were used to analyze the data. Results: The RST group improved sprint performances with moderate to large effect sizes (0.76–1.41) across all distances, both within and between groups (>92% beneficial effect likelihood). Conversely, there were no clear improvements in the UST and control groups. The RST evoked the largest improvements over short distances (6%–8%) and was strongly associated with increased maximum horizontal force capacities (r = .9). Players with a preintervention deficit in force capacity appeared to benefit the most from RST. Conclusions: Four weeks of heavy RST led to superior improvements in short-sprint performance compared with UST among adolescent soccer players. Heavy RST, using a load individually selected to maximize horizontal power, is therefore highly recommended as a method to improve sprint acceleration in youth athletes.
... ds were standardized to a specific velocity loss (VL) from maximal velocity for each player in the intervention groups. As surface friction can highly influence the net resistance provided by the sled, measuring running velocity instead of the absolute load is more accurate for standardizing a specific target stimulus (Cahill, Oliver, et al., 2019;M. R. Cross et al., 2019). ...
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Despite efforts to intervene, hamstring muscle injuries (HMI) continue to be one of the largest epidemiological burdens in professional football. The injury mechanism takes place dominantly during sprinting, but also other scenarios have been observed, such as overstretching actions, jumps, and change of directions. The main biomechanical roles of the hamstring muscles are functioning as an accelerator of center-of-mass (i.e., contributing to horizontal force production), and stabilizing the pelvis and knee joint. Multiple extrinsic and intrinsic risk factors have been identified, portraying the multifactorial nature of the HMI. Furthermore, these risk factors can vary substantially between players, portraying the importance of individualized approaches. However, there is a lack of multifactorial and individualized approaches assessed for validity in literature. Thus, the overarching aim of this doctoral thesis was to explore if a specific multifactorial and individualized approach can improve upon the ongoing HMI risk reduction protocols, and thus, further reduce the HMI risk in professional football players. This was done following the Team-sport Injury Prevention model (TIP model), where the target is to evaluate the current injury burden, identify possible solutions, and intervene. The thesis comprised of three themes within professional football, I) evaluating and identifying HMI risk (completed via assessing the current epidemiological HMI situation and the association between HMI injuries and a novel hamstring screening protocol), II) improving horizontal force capacity (completed via testing if maximal theoretical horizontal force (F0) can be improved via heavy resisted sprint training), and III) developing and conducting a multifactorial and individualized training for HMI risk reduction (completed via introducing and conducting a training intervention). The conclusions from theme I were that the HMI burden continues to be high (14.1 days absent per 1000 hours of football exposure), no tests from the screening protocol were associated with an increased HMI risk when including all injuries from the season (n = 17, p > 0.05), and that lower F0 was significantly associated with increased HMI risk when including injuries between test rounds one and two (~90 days, n =14, hazard ratio: 4.02 (CI95% 1.08 to 15.0), p = 0.04). For theme II, the players initial pre-season level of F0 was significantly associated with adaptation potential after 11 weeks of heavy resisted sprint training during the pre-season (r = -0.59, p < 0.05). The heavy resisted sprint load leading to a ~50% velocity loss induced the largest improvements in sprint mechanical output and sprint performance variables. For theme III, no intervention results could be presented within this document due to the Covid-19 pandemic leading to the intervention being postponed. However, a protocol paper was published, describing in detail the intervention approach that will be used outside the scope of the thesis. In future studies, larger sample size studies are needed to support the development of more advanced HMI risk reduction models. Such models may allow practitioners to identify risk on an individual level instead of a group level. Furthermore, constant development of more specific, reliable, and accessible risk assessment tests should be promoted that can be frequently tested throughout the football season. Finally, based on the results of theme II, individualization of a specific training stimulus should be promoted in team settings.
... [2][3][4][5] RST devices predominantly provide a concentric overload to the musculoskeletal system by overloading horizontal force production during a sprint. [6][7][8] RST devices are often used by strength and conditioning coaches as an adjunct to gym-based resistance training. However, it may be that eccentric towing devices provide a better form of overload for the athlete given that: (1) a shift in fibre type towards fast twitch type IIb with eccentric training can yield improvements in high speed concentric power [9][10][11] ; (2) an increase in leg spring stiffness from eccentric training can yield improvements in higher stride frequency due to a decreased ground contact time 6,12 ; and, (3) an eccentric leg press motion has been shown to improve both jump height and sprint time. ...
Horizontal eccentric towing (HET) is a novel modality that delivers an eccentric overload to the musculature as an athlete attempts to move forward in a sprint stance whilst being pulled backwards. A device, called the HET, has been developed to automate this movement. Similar to a winch retracting an anchor on a boat, the HET device pulls an athlete that is tethered inwards and the athlete must resist this motion in a maximal manner, whilst maintaining a sprint stance. The HET device provides an isokinetic training modality by towing athletes inwards at a constant velocity. The HET device is operated by an electric 10 kW AC synchronous servo gearmotor. The motor is controlled by a variable speed drive (VSD) and programmable logic controller (PLC), which allow for accurate speed, position and torque control. A touchscreen PC runs the user interface displaying real-time force and speed measures. The HET device can produce a maximum towing force of 2.8 kN at ground velocities of up to 3.58 m/s. There is a separate safety PLC that triggers a safety-rated brake system when the E-stop buttons are pushed. This is paramount for athlete safety. In this technical note, the components used in the construction of the HET device will be detailed and insights into a novel stimulus will be offered, as well as a guide to develop and automate similar eccentric movements.
... Commonly, load is either standardized according to % of BM or a velocitybased training approach [14,15]. To control for both internal (relative strength) and external (friction) factors and improve individualization of load, determining loading parameters according to the velocity-based training approach is likely a more effective approach [24]. Load-velocity tests were completed under one unloaded and three loaded conditions with one sprint per load (50%, 75%, 100% of BM) for the RESISTED group outlined in previous literature [25]. ...
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We tested the hypothesis that the degree of adaptation to highly focused sprint training at opposite ends of the sprint Force-Velocity (FV) spectrum would be associated with initial sprint FV-profile in rugby athletes. Training-induced changes in sprint FV-profiles were computed before and after an 8-week in-season resisted or assisted sprint training protocol, including a 3-week taper. Professional male rugby players (age: 18.9 ± 1.0 years; body-height: 1.9 ± 0.0 m; body-mass: 88.3 ± 10.0 kg) were divided into two groups based on their initial sprint FV-profiles: 1) heavy sled training (RESISTED, N = 9, velocity loss 70-80%), and 2) assisted acceleration training (ASSISTED, N = 12, velocity increase 5-10%). A total of 16 athletes were able to finish all required measurements and sessions. According to the hypothesis, a significant correlation was found between initial sprint FV-profile and relative change in sprint FV-profile (RESISTED: r = -0.95, p<0.01, ASSISTED: r = -0.79, p<0.01). This study showed that initial FV-properties influence the degree of mechanical response when training at different ends of the FV-spectrum. Practitioners should consider utilizing the sprint FV-profile to improve the individual effectiveness of resisted and assisted sprint training programs in high-level rugby athletes.
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In this study, we assessed the acute kinematic effects of different sled load conditions (unloaded and at 10%, 20%, 30% decrement from maximum velocity (Vdec)) in different sporting populations. It is well-known that an athlete’s kinematics change with increasing sled load. However, to our knowledge, the relationship between the different loads in resisted sled sprinting (RSS) and kinematic characteristics is unknown. Thirty-three athletes (sprinters n = 10; team sport athletes n = 23) performed a familiarization session (day 1), and 12 sprints at different loads (day 2) over a distance of 40 m. Sprint time and average velocity were measured. Sagittal-plane high-speed video data was recorded for early acceleration and maximum velocity phase and joint angles computed. Loading introduced significant changes to hip, knee, ankle, and trunk angle for touch-down and toe-off for the acceleration and maximum velocity phase (p < 0.05). Knee, hip, and ankle angles became more flexed with increasing load for all groups and trunk lean increased linearly with increasing loading conditions. The results of this study provide coaches with important information that may influence how RSS is employed as a training tool to improve sprint performance for acceleration and maximal velocity running and that prescription may not change based on sporting population, as there were only minimal differences observed between groups. The trunk lean increase was related to the heavy loads and appeared to prevent athletes to reach mechanics that were truly reflective of maximum velocity sprinting. Lighter loads seem to be more adequate to not provoke changes in maxV kinematics. However, heavy loading extended the distance over which it is possible to train acceleration.
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Background Sprint performance is an essential skill to target within soccer, which can be likely achieved with a variety of methods, including different on-field training options. One such method could be heavy resisted sprint training. However, the effects of such overload on sprint performance and the related kinetic changes are unknown in a professional setting. Another unknown factor is whether violating kinematic specificity via heavy resistance will lead to changes in unloaded sprinting kinematics. We investigated whether heavy resisted sled training (HS) affects sprint performance, kinetics, sagittal plane kinematics, and spatiotemporal parameters in professional male soccer players. Methods After familiarization, a nine-week training protocol and a two-week taper was completed with sprint performance and force-velocity (FV) profiles compared before and after. Out of the two recruited homogenous soccer teams ( N = 32, age: 24.1 ± 5.1 years: height: 180 ± 10 cm; body-mass: 76.7 ± 7.7 kg, 30-m split-time: 4.63 ± 0.13 s), one was used as a control group continuing training as normal with no systematic acceleration training (CON, N = 13), while the intervention team was matched into two HS subgroups based on their sprint performance. Subgroup one trained with a resistance that induced a 60% velocity decrement from maximal velocity ( N = 10, HS60%) and subgroup two used a 50% velocity decrement resistance ( N = 9, HS50%) based on individual load-velocity profiles. Results Both heavy resistance subgroups improved significantly all 10–30-m split times ( p < 0.05, d = − 1.25; −0.62). Post-hoc analysis showed that HS50% improved significantly more compared to CON in 0–10-m split-time ( d = 1.03) and peak power ( d = 1.16). Initial maximal theoretical horizontal force capacity (F0) and sprint FV-sprint profile properties showed a significant moderate relationship with F0 adaptation potential ( p < 0.05). No significant differences in sprinting kinematics or spatiotemporal variables were observed that remained under the between-session minimal detectable change. Conclusion With appropriate coaching, heavy resisted sprint training could be one pragmatic option to assist improvements in sprint performance without adverse changes in sprinting kinematics in professional soccer players. Assessing each player’s initial individual sprint FV-profile may assist in predicting adaptation potential. More studies are needed that compare heavy resisted sprinting in randomized conditions.
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Sprint performance is an essential skill to target within soccer. However, time-consuming intervention models could be easily rejected by coaches. Therefore, alternative and efficient field training options are warranted. One such method could be heavy resisted sprint training. However, it is unknown whether such overload will be efficient in assisting increases in sprint performance in a professional setting, and whether violating kinematic specificity via heavy loading will lead to changes in unloaded sprinting kinematics. Thus, we investigated whether heavy resisted training affects sprint performance and sagittal plane kinematics. Training-induced changes in sprint FV-profiles were computed before and after the 9-week, 2 sessions x week protocol. Out of the two recruited teams (N = 32, age: 24.1 ± 5.0 years: height: 180 ± 10 cm; body-mass: 76.7 ± 7.7 kg), one was used as a control group continuing training as normal (CON, N = 13), while the experimental team was divided into two subgroups based on their initial sprint performance: 1) Heavy sled training with the 60% velocity drop (N = 10) and 2) 50% velocity drop load (N = 9). Both experimental groups improved significantly all 0-30-m split times (p < 0.05, d = -0.62 – -1.25), with post-hoc showing HS50% improving significantly compared to CON in 0-10-m split (d = 1.03) and Pmax (d = 1.16). No differences in sprinting kinematics were observed. With appropriate coaching, heavy sled training could be a pragmatic option to assist improvements in sprint performance without adverse changes in sprinting kinematics in professional soccer players.
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Background: Sprinting is key in the development and final results of competitions in a range of sport disciplines, both individual (e.g., athletics) and team sports. Resisted sled training (RST) might provide an effective training method to improve sprinting, in both the acceleration and the maximum-velocity phases. However, substantial discrepancies exist in the literature regarding the influence of training status and sled load prescription in relation to the specific components of sprint performance to be developed and the phase of sprint. Objectives: Our objectives were to review the state of the current literature on intervention studies that have analyzed the effects of RST on sprint performance in both the acceleration and the maximum-velocity phases in healthy athletes and to establish which RST load characteristics produce the largest improvements in sprint performance. Methods: We performed a literature search in PubMed, SPORTDiscus, and Web of Science up to and including 9 January 2018. Peer-reviewed studies were included if they met all the following eligibility criteria: (1) published in a scientific journal; (2) original experimental and longitudinal study; (3) participants were at least recreationally active and towed or pulled the sled while running at maximum intensity; (4) RST was one of the main training methods used; (5) studies identified the load of the sled, distance covered, and sprint time and/or sprint velocity for both baseline and post-training results; (6) sprint performance was measured using timing gates, radar gun, or stopwatch; (7) published in the English language; and (8) had a quality assessment score > 6 points. Results: A total of 2376 articles were found. After filtering procedures, only 13 studies were included in this meta-analysis. In the included studies, 32 RST groups and 15 control groups were analyzed for sprint time in the different phases and full sprint. Significant improvements were found between baseline and post-training in sprint performance in the acceleration phase (effect size [ES] 0.61; p = 0.0001; standardized mean difference [SMD] 0.57; 95% confidence interval [CI] - 0.85 to - 0.28) and full sprint (ES 0.36; p = 0.009; SMD 0.38; 95% CI - 0.67 to - 0.10). However, non-significant improvements were observed between pre- and post-test in sprint time in the maximum-velocity phase (ES 0.27; p = 0.25; SMD 0.18; 95% CI - 0.49 to 0.13). Furthermore, studies that included a control group found a non-significant improvement in participants in the RST group compared with the control group, independent of the analyzed phase. Conclusions: RST is an effective method to improve sprint performance, specifically in the early acceleration phase. However, it cannot be said that this method is more effective than the same training without overload. The effect of RST is greatest in recreationally active or trained men who practice team sports such as football or rugby. Moreover, the intensity (load) is not a determinant of sprint performance improvement, but the recommended volume is > 160 m per session, and approximately 2680 m per week, with a training frequency of two to three times per week, for at least 6 weeks. Finally, rigid surfaces appear to enhance the effect of RST on sprint performance.
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Aims In the current study we investigated the effects of resisted sprint training on sprinting performance and underlying mechanical parameters (force-velocity-power profile) based on two different training protocols: (i) loads that represented maximum power output (Lopt) and a 50% decrease in maximum unresisted sprinting velocity and (ii) lighter loads that represented a 10% decrease in maximum unresisted sprinting velocity, as drawn from previous research (L10). Methods Soccer [n = 15 male] and rugby [n = 21; 9 male and 12 female] club-level athletes were individually assessed for horizontal force-velocity and load-velocity profiles using a battery of resisted sprints, sled or robotic resistance respectively. Athletes then performed a 12-session resisted (10 × 20-m; and pre- post-profiling) sprint training intervention following the L10 or Lopt protocol. Results Both L10 and Lopt training protocols had minor effects on sprinting performance (average of -1.4 to -2.3% split-times respectively), and provided trivial, small and unclear changes in mechanical sprinting parameters. Unexpectedly, Lopt impacted velocity dominant variables to a greater degree than L10 (trivial benefit in maximum velocity; small increase in slope of the force-velocity relationship), while L10 improved force and power dominant metrics (trivial benefit in maximal power; small benefit in maximal effectiveness of ground force orientation). Conclusions Both resisted-sprint training protocols were likely to improve performance after a short training intervention in already sprint trained athletes. However, widely varied individualised results indicated that adaptations may be dependent on pre-training force-velocity characteristics.
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PurposeWe sought to compare force–velocity relationships developed from unloaded sprinting acceleration to that compiled from multiple sled-resisted sprints. Methods Twenty-seven mixed-code athletes performed six to seven maximal sprints, unloaded and towing a sled (20–120% of body-mass), while measured using a sports radar. Two methods were used to draw force–velocity relationships for each athlete: A multiple trial method compiling kinetic data using pre-determined friction coefficients and aerodynamic drag at maximum velocity from each sprint; and a validated single trial method plotting external force due to acceleration and aerodynamic drag and velocity throughout an acceleration phase of an unloaded sprint (only). Maximal theoretical force, velocity and power were determined from each force–velocity relationship and compared using regression analysis and absolute bias (± 90% confidence intervals), Pearson correlations and typical error of the estimate (TEE). ResultsThe average bias between the methods was between − 6.4 and − 0.4%. Power and maximal force showed strong correlations (r = 0.71 to 0.86), but large error (TEE = 0.53 to 0.71). Theoretical maximal velocity was nearly identical between the methods (r = 0.99), with little bias (− 0.04 to 0.00 m s−1) and error (TEE = 0.12). Conclusions When horizontal force or power output is considered for a given speed, resisted sprinting is similar to its associated phase during an unloaded sprint acceleration [e.g. first steps (~ 3 m s−1) = heavy resistance]. Error associated with increasing loading could be resultant of error, fatigue, or technique, and more research is needed. This research provides a basis for simplified assessment of optimal loading from a single unloaded sprint.
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Purpose: The effects of different loads on kinematic and kinetic variables during sled towing were investigated with the aim to identify the optimal overload for this specific sprint training. Methods: Thirteen male sprinters (100m PB: 10.91±0.14 s) performed 5 maximal trials over a 20m distance in the following conditions: unloaded (UL) and with loads from +15% to +40% of the athlete's body mass (BM). In these calculations the sled mass and friction were taken into account. Contact and flight times (CT, FT), step length (SL), horizontal hip velocity (vh) and relative angles of hip, knee and ankle (at touch-down and take-off) were measured step-by-step. In addition, the horizontal force (Fh) and power (Ph) and the maximal force (Fh0) and power (Ph0) were calculated. Results: vh, FT and SL decreased while CT increased with increasing load (P < .001). These variables changed significantly also as a function of the step number (P < .01) except between the two last steps. No differences were observed in Fh among loads but Fh was larger in sled towing compared to UL. Ph was unaffected by load up +20%BM but decreased with larger loads. Fh0 and Ph0 were achieved at +20%BM. Up to +20%BM no significant effects on joint angles were observed at touch-down and take-off, while at loads >+30%BM joint angles tend to decrease. Conclusions: The +20%BM condition represents the optimal overload for peak power production: at this load sprinters reach their highest power without significant changes in their running technique (e.g. joint angles).
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Purpose: To ascertain whether force-velocity-power relationships could be compiled from a battery of sled-resisted overground sprints and to clarify and compare the optimal loading conditions for maximizing power production for different athlete cohorts. Methods: Recreational mixed-sport athletes (n = 12) and sprinters (n = 15) performed multiple trials of maximal sprints unloaded and towing a selection of sled masses (20-120% body mass [BM]). Velocity data were collected by sports radar, and kinetics at peak velocity were quantified using friction coefficients and aerodynamic drag. Individual force-velocity and power-velocity relationships were generated using linear and quadratic relationships, respectively. Mechanical and optimal loading variables were subsequently calculated and test-retest reliability assessed. Results: Individual force-velocity and power-velocity relationships were accurately fitted with regression models (R2> .977, P < .001) and were reliable (ES = 0.05-0.50, ICC = .73-.97, CV = 1.0-5.4%). The normal loading that maximized peak power was 78% ± 6% and 82% ± 8% of BM, representing a resistance of 3.37 and 3.62 N/kg at 4.19 ± 0.19 and 4.90 ± 0.18 m/s (recreational athletes and sprinters, respectively). Optimal force and normal load did not clearly differentiate between cohorts, although sprinters developed greater maximal power (17.2-26.5%, ES = 0.97-2.13, P < .02) at much greater velocities (16.9%, ES = 3.73, P < .001). Conclusions: Mechanical relationships can be accurately profiled using common sled-training equipment. Notably, the optimal loading conditions determined in this study (69-96% of BM, dependent on friction conditions) represent much greater resistance than current guidelines (~7-20% of BM). This method has potential value in quantifying individualized training parameters for optimized development of horizontal power.
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Understanding the impact of friction in sled sprinting allows the quantification of kinetic outputs and the effective loading experienced by the athlete. This study assessed changes in the coefficient of friction (µk) of a sled sprint-training device with changing mass and speed to provide a means of quantifying effective loading for athletes. A common sled equipped with a load cell was towed across an athletics track using a motorised winch under variable sled mass (33.1–99.6 kg) with constant speeds (0.1 and 0.3 m · s−1), and with constant sled mass (55.6 kg) and varying speeds (0.1–6.0 m · s−1). Mean force data were analysed, with five trials performed for each condition to assess the reliability of measures. Variables were determined as reliable (ICC > 0.99, CV < 4.3%), with normal-force/friction-force and speed/coefficient of friction relationships well fitted with linear (R2 = 0.994–0.995) and quadratic regressions (R2 = 0.999), respectively (P < 0.001). The linearity of composite friction values determined at two speeds, and the range in values from the quadratic fit (µk = 0.35–0.47) suggested µk and effective loading were dependent on instantaneous speed on athletics track surfaces. This research provides a proof-of-concept for the assessment of friction characteristics during sled towing, with a practical example of its application in determining effective loading and sled-sprinting kinetics. The results clarify effects of friction during sled sprinting and improve the accuracy of loading applications in practice and transparency of reporting in research.
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The ability of the human body to generate maximal power is linked to a host of performance outcomes and sporting success. Power-force-velocity relationships characterize limits of the neuromuscular system to produce power, and their measurement has been a common topic in research for the past century. Unfortunately, the narrative of the available literature is complex, with development occurring across a variety of methods and technology. This review focuses on the different equipment and methods used to determine mechanical characteristics of maximal exertion human sprinting. Stationary cycle ergometers have been the most common mode of assessment to date, followed by specialized treadmills used to profile the mechanical outputs of the limbs during sprint running. The most recent methods use complex multiple-force plate lengths in-ground to create a composite profile of over-ground sprint running kinetics across repeated sprints, and macroscopic inverse dynamic approaches to model mechanical variables
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The purpose of this study was to assess validity and reliability of sprint performance outcomes measured with an iPhone application (named: MySprint) and existing field methods (i.e. timing photocells and radar gun). To do this, 12 highly trained male sprinters performed 6 maximal 40-m sprints during a single session which were simultaneously timed using 7 pairs of timing photocells, a radar gun and a newly developed iPhone app based on high-speed video recording. Several split times as well as mechanical outputs computed from the model proposed by Samozino et al. [(2015). A simple method for measuring power, force, velocity properties, and mechanical effectiveness in sprint running. Scandinavian Journal of Medicine & Science in Sports.] were then measured by each system, and values were compared for validity and reliability purposes. First, there was an almost perfect correlation between the values of time for each split of the 40-m sprint measured with MySprint and the timing photocells (r=0.989–0.999, standard error of estimate=0.007–0.015 s, intraclass correlation coefficient (ICC)=1.0). Second, almost perfect associations were observed for the maximal theoretical horizontal force (F0), the maximal theoretical velocity (V0), the maximal power (Pmax) and the mechanical effectiveness (DRF – decrease in the ratio of force over acceleration) measured with the app and the radar gun (r= 0.974–0.999, ICC=0.987–1.00). Finally, when analysing the performance outputs of the six different sprints of each athlete, almost identical levels of reliability were observed as revealed by the coefficient of variation (MySprint: CV=0.027–0.14%; reference systems: CV=0.028–0.11%). Results on the present study showed that sprint performance can be evaluated in a valid and reliable way using a novel iPhone app.
“Should have broad appeal in many kinds of industry, ranging from automotive to computers-basically any organization concerned with products having moving parts!" -David A. Rigney, Materials Science and Engineering Department, Ohio State University, Columbus, USA In-Depth Coverage of Frictional Concepts Friction affects so many aspects of daily life that most take it for granted. Arguably, mankind’s attempt to control friction dates back to the invention of the wheel. Friction Science and Technology: From Concepts to Applications, Second Edition presents a broad, multidisciplinary overview of the constantly moving field of friction, spanning the history of friction studies to the evolution of measurement instruments. It reviews the gamut of friction test methods, ranging from simple inclined plans to sophisticated laboratory tribometers. The book starts with introductory concepts about friction and progressively delves into the more subtle fundamentals of surface contact, use of various lubricants, and specific applications such as brakes, piston rings, and machine components. Includes American Society of Testing and Management (ASTM) Standards This volume covers multiple facets of friction, with numerous interesting and unusual examples of friction-related technologies not found in other tribology books. These include: •Friction in winter sports •Friction of touch and human skin •Friction of footware and biomaterials •Friction drilling of metals •Friction of tires and road surfaces Describing the tools of the trade for friction research, this edition enables engineers to purchase or build their own devices. It also discusses frictional behavior of a wide range of materials, coatings, and surface treatments, both traditional and advanced, such as thermally oxidized titanium alloys, nanocomposites, ultra-low friction films, laser-dimpled ceramics, and carbon composites. Even after centuries of study, friction continues to conceal its subtle origins, especially in practical engineering situations in which surfaces are exposed to complex and changing environments. Authored by a field specialist with more than 30 years of experience, this one-stop resource discusses all aspects of friction, from its humble beginnings to its broad application for modern engineers.
We aimed to clarify the mechanical determinants of sprinting performance during acceleration and maximal speed phases of a single sprint, using ground reaction forces (GRFs). While 18 male athletes performed a 60-m sprint, GRF was measured at every step over a 50-m distance from the start. Variables during the entire acceleration phase were approximated with a fourth-order polynomial. Subsequently, accelerations at 55%, 65%, 75%, 85%, and 95% of maximal speed, and running speed during the maximal speed phase were determined as sprinting performance variables. Ground reaction impulses and mean GRFs during the acceleration and maximal speed phases were selected as independent variables. Stepwise multiple regression analysis selected propulsive and braking impulses as contributors to acceleration at 55%-95% (β > 0.724) and 75%-95% (β > 0.176), respectively, of maximal speed. Moreover, mean vertical force was a contributor to maximal running speed (β = 0.481). The current results demonstrate that exerting a large propulsive force during the entire acceleration phase, suppressing braking force when approaching maximal speed, and producing a large vertical force during the maximal speed phase are essential for achieving greater acceleration and maintaining higher maximal speed, respectively.